Efficient coding and decoding of audio signals often includes reducing the amount of audio-related data to be encoded, transmitted and/or decoded based on psycho-acoustic principles. This includes for example discarding so-called masked audio content which is present in an audio signal but not perceivable by a listener. Alternatively or in addition, the bandwidth of an audio signal to be encoded may be limited, while only keeping respectively calculating some information on its higher frequency content without actually encoding such higher frequency content directly. The band-limited signal is then encoded and transmitted (or stored) together with said higher frequency information, the latter requiring less resources than directly encoding also the higher frequency content.
Spectral Band Replication (SBR) in HE-AAC (High Efficiency-Advanced Audio Coding) and Spectral Extension (SPX) in Dolby Digital Plus are two examples for audio coding systems which approximate or reconstruct a high frequency component of an audio signal based on a low frequency component of the audio signal and based on additional side information (also referred to as higher frequency information). In the following, reference is made to the SPX scheme of Dolby Digital Plus. It should be noted, however, that the methods and systems described in the present document are applicable to High Frequency Reconstruction techniques in general, including SBR in HE-AAC.
The determination of the side information in an SPX based audio encoder is typically subject to significant computational complexity. By way of example, the determination of the side information may require around 50% of the total computational resources of the audio encoder. The present document describes methods and systems which allow reducing the computational complexity of SPX based audio encoders. In particular, the present document describes methods and systems which allow reducing the computational complexity for performing tonality calculations in the context of SPX based audio encoders (wherein the tonality calculations may account for around 80% of the computational complexity used for determining the side information).